1. Field of the Invention
This invention relates to an apparatus for processing a digital audio signal. In addition, this invention relates to a method of processing a digital audio signal. Furthermore, this invention relates to a recording medium which stores a computer program for processing a digital audio signal.
2. Description of the Related Art
According to the CD (compact disc) standards, an analog audio signal is converted into a digital audio signal at a sampling frequency fs of 44.1 kHz and a quantization bit number of 16. Generally, a higher sampling frequency and a larger quantization bit number provide a better tone quality. There is a DVD (digital versatile disc) family having a DVD-video and a DVD-audio. According to the DVD-video standards and the DVD-audio standards, the sampling frequency fs is equal to 48 kHz, 96 kHz, or 192 kHz while the quantization bit number is equal to, for example, 24.
Japanese patent application publication number 11-126097/1999 discloses an apparatus for processing a digital audio signal. The apparatus in Japanese application 11-126097 includes a bit converter receiving a first digital audio signal (an input digital audio signal) having a sequence of 16-bit samples which relates to a sampling frequency of 44.1 kHz. The bit converter changes every 16-bit sample of the input digital audio signal into a corresponding 24-bit sample. Thus, the first digital audio signal is converted into a second digital audio signal having a sequence of 24-bit samples which relates to a sampling frequency of 44.1 kHz. The apparatus also includes a sampling-rate converter receiving the second digital audio signal. The sampling-rate converter over-samples the second digital audio signal, thereby generating a third digital audio signal having a sequence of 24-bit samples which relates to a sampling frequency of 96 kHz. The apparatus in Japanese application 11-126097 further includes a higher-harmonic generator receiving the third digital audio signal.
The higher-harmonic generator in the apparatus of Japanese application 11-126097 compares every sample of the third digital audio signal with the immediately-preceding sample thereof to generate a binary signal (a 0–1 signal) representing the comparison result. When the signal level represented by the present sample is greater than that represented by the immediately-preceding sample, the comparison-result signal is “0”. Otherwise, the comparison-result signal is “1”. In response to the comparison-result signal, the higher-harmonic generator detects samples of the third digital audio signal which correspond to signal-level peaks and valleys. The higher-harmonic generator implements a pattern matching procedure for the 0–1 pattern represented by the comparison-result signal which occurs during the time interval from every signal-level peak to a following signal-level valley, and also the 0–1 pattern represented by the comparison-result signal which occurs during the time interval from every signal-level valley to a following signal-level peak. Specifically, the higher-harmonic generator decides which of predetermined reference patterns the peak-to-valley 0–1 pattern and the valley-to-peak 0–1 pattern match with. On the basis of the matching reference patterns, the higher-harmonic generator produces a sequence of 24-bit samples of a digital emphasis signal which relates to a sampling frequency of 96 kHz.
The apparatus in Japanese application 11-126097 includes an adder which combines the third digital audio signal and the digital emphasis signal into a fourth digital audio signal. The fourth digital audio signal is wider in frequency band than the input digital audio signal (the first digital audio signal). The apparatus records the fourth digital audio signal on a recording medium.
In the apparatus of Japanese application 11-126097, each of the reference patterns relates to the number of samples in the time interval from the signal-level peak to the following signal-level valley or the time interval from the signal-level valley to the following signal-level peak. Therefore, the pattern matching detects the number of samples in the time interval from every signal-level peak to the following signal-level valley and the number of samples in the time interval from every signal-level valley to the following signal-level peak. A coefficient is calculated from each of the detected sample numbers. The difference between each peak-corresponding or valley-corresponding sample of the third digital audio signal and the immediately-preceding sample thereof is multiplied by the calculated coefficient. The multiplication result is added to samples of the third digital audio signal which are temporally near the peak-corresponding sample. The multiplication result is subtracted from samples of the third digital audio signal which are temporally near the valley-corresponding sample. As a result, the third digital audio signal is converted or modified into the fourth digital audio signal.
Japanese patent application publication number 11-144382/1999 discloses an apparatus for processing a digital audio signal. The apparatus in Japanese application 11-144382 includes a bit converter receiving a first digital audio signal (an input digital audio signal) having a sequence of 16-bit samples which relates to a sampling frequency of 44.1 kHz. The bit converter changes every 16-bit sample of the input digital audio signal into a corresponding 24-bit sample. Thus, the first digital audio signal is converted into a second digital audio signal having a sequence of 24-bit samples which relates to a sampling frequency of 44.1 kHz. The apparatus also includes a sampling-rate converter receiving the second digital audio signal. The sampling-rate converter over-samples the second digital audio signal, thereby generating a third digital audio signal having a sequence of 24-bit samples which relates to a sampling frequency of 96 kHz. The apparatus in Japanese application 11-144382 further includes a trapezoid waveform generator receiving the third digital audio signal.
The trapezoid waveform generator in the apparatus of Japanese application 11-144382 compares every sample of the third digital audio signal with immediately-preceding sample thereof to generate a binary signal (a 0–1 signal) representing the comparison result. When the signal level represented by the present sample is greater than that represented by the immediately-preceding sample, the comparison-result signal is “0”. Otherwise, the comparison-result signal is “1”. In response to the comparison-result signal, the trapezoid waveform generator detects samples of the third digital audio signal which correspond to signal-level peaks and valleys. On the basis of the detected peak-corresponding samples and the detected valley-corresponding samples, the trapezoid waveform generator produces a sequence of 24-bit samples of a corrective digital signal which relates to a sampling frequency of 96 kHz. The corrective digital signal represents trapezoid waveforms centered at the detected peak-corresponding samples and the detected valley-corresponding samples.
The apparatus in Japanese application 11-144382 includes a high pass filter which receives the corrective digital signal. The high pass filter processes the corrective digital signal into a digital emphasis signal which has a sequence of 24-bit samples, and which relates to a sampling frequency of 96 kHz. The apparatus further includes an adder which combines the third digital audio signal and the digital emphasis signal into a fourth digital audio signal. The fourth digital audio signal is wider in frequency band than the input digital audio signal (the first digital audio signal). The apparatus records the fourth digital audio signal on a recording medium.